It is well known in the art that there are many different types of the contact lens including a soft contact lens, rigid contact lens, and color contact lens for fashion. Now, it is reported in Japan that almost 16 million people wear contact lenses. Further, the prescribed wearing period of the contacts may be ranged from a day to a few years.
It is reported in many European and American countries from about 1974 that the corneal infection due to acanthamoeba had been occurred among contact lens wearers, and now the mechanism of acanthamoeba keratitis is evident.
Acanthamoeba is a genus of amoebae, one of the most common protozoa in soil, and also frequently found in fresh water and in river, lake, and pound or other habitats. Acanthamoeba ingests microorganisms as nutrient and proliferated. Upon number of microorganisms are reduced, it takes a form of cyst to halt the proliferation. Further getting worse the environment, it will die.
The contact lens of soft type is made of a material higher in its ability to hold water so that the lenses are apt to be contaminated by the deposition and colonization of acanthamoeba.
In this connection, it is believed that the risk factors associated with acanthamoeba keratitis are higher in the soft contact lens wearer.
The majority of the soft contact lens wearers are prescribed some type of frequent replacement schedule. With a true daily wear disposable schedule, a brand new pair of lenses is used each day. However, actually, they may be worn continuously after the prescribed schedule had expired (for example 4 or 5 days or more).
After removed the contact lenses, they are immersed within tap water or multipurpose solution (referred herein below to as MPS) within the lens storage case. When it is intended to put them on, they are picked up from the case.
When the lenses are handled by fingers and hands contaminated by any bacteria or acanthamoeba, the surface of the lenses and the solution within the case may also be contaminated. In this connection, the lenses stored in the case will further be contaminated.
The infection has been associated with penetrating corneal trauma. The main cause of the infection is to wear the contaminated contact lenses. The basic countermeasures to be taken for preventing the infection are to handle the lens sterilely and appropriately.
Although the bacteria and acanthamoeba can easily be killed by thermal disinfection, the heat energy required for the disinfection will distort the lens to destroy the function thereof.
Organic bacteriocides such as alcoholic, halogenic, and phenolic bacteriocides or antimicrobial agents have toxicity to the cells of the eyes so that using such bacteriocidal additives in MPS may be problematic and cannot be put into practical use. Although the MPS including polyvalent cationic chelate has been proposed (see patent 1 listed hereinbelow), this MPS does not have sufficient effect for killing on acanthamoeba with the predetermined short period.
Today, we have no sovereign remedy against corneal infection due to acanthamoeba, and it is very difficult to treat it.
[patent 1] Japanese Laid Open Public Disclosure 2005-177515